Method for transmitting data from sensor to a control unit, and a corresponding sensor and control unit

ABSTRACT

A method of data transmission from a sensor to a control unit and a sensor and control unit are described, the sensor transmitting both differential values and absolute values. The differential values are analyzed in the control unit to perform the function provided, and the absolute values are analyzed for the plausibility check of the function.

FIELD OF THE INVENTION

[0001] The present invention relates to a method for data transmissionfrom a sensor to a control unit as well as a corresponding sensor and acorresponding control unit for restraining systems.

BACKGROUND INFORMATION

[0002] Methods of transmitting data from at least one sensor to acontrol unit, in particular in connection with restraining systems, inwhich the data is transmitted using current modulation via a two-wireline from the sensors to a control unit in accordance with apredetermined format, are known from German Published Patent ApplicationNos. 101 14 504 and 101 49 332. The format of the data transmissionprovides a fixed assignment of parts of the value range available forthe data transmission to the sensor values, a first part of the valuerange being used for sensor values, i.e., useful data, a second part forstatus and error messages, and a third part for sensor identificationdata. These parts are separated from one another and follow one anotherduring transmission.

[0003] Furthermore, in many applications, in particular in connectionwith restraining systems, the use of pressure sensors, which aredistributed in the vehicle and are connected via such an interface oranother interface to a central control unit, is known.

SUMMARY OF THE INVENTION

[0004] Through the transmission of an absolute pressure value forpressure sensors which transmit a differential pressure as the usefuldata, performing a function verification of these pressure sensors, andtherefore error detection, is advantageously made possible in a systemwhich includes at least two sensors and a central control unit.

[0005] It is especially advantageous that fault-free functioning of allpressure sensors located in the system is provided using cost-effectivesoftware implementation without additional hardware expense.Introduction of error detection in such a sensor system is thus madepossible through a change in the software alone, without a change in thesystem.

[0006] It is especially advantageous to transmit the absolute pressurevalue instead of the differential measured pressure value in the courseof the initialization phase of the system. In this way, checking thepressure sensors before beginning to operate the system is madepossible.

[0007] In an especially simple way, the known current-based two-wireinterface is used for data transmission.

[0008] According to a further aspect of the present invention, checkingof pressure sensors during running operation in a sensor system having acentral control unit is made possible in an especially advantageous wayif the transmission of absolute pressure values during running operationof the system is mixed with the transmission of differential pressurevalues.

[0009] It has been shown to be especially advantageous in this case, foran interface whose value range for data transmission is divided into atleast two parts, to perform the absolute pressure values in the part ofthe value range which is not available for the sensor measured values,i.e., the differential pressure values. In this way, mutual influence ofthe measured value and absolute pressure value is effectively avoided.

[0010] It is especially advantageous, for the data format described inthe related art initially cited, to code the absolute pressure values ina value range which lies outside the useful signal value range, theabsolute pressure values being assigned additional identification codes.Both the identification code and the data word are located outside thevalue range of the differential measured pressure values, so thatadvantageously there can be no confusion of the individual pressurevalues.

[0011] The absolute pressure values are advantageously transmitted onlyas long as there is no significant signal change of the differentialpressure. As soon as such a change occurs, the running absolute pressuretransmission is stopped and the system switches to differential pressuretransmission. In this way, in particular for use in connection withrestraining systems, the system operation and its intended result arenot impaired.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 shows an overview of a sensor system having a centralcontrol unit.

[0013]FIG. 2 shows a first flow chart, the procedure for datatransmission in connection with pressure sensors, preferably inrestraining systems, is outlined on the basis of flow charts.

[0014]FIG. 3 shows a second flow chart, the procedure for datatransmission in connection with pressure sensors, preferably inrestraining systems, is outlined on the basis of flow charts.

[0015]FIG. 4 shows a third flow chart, the procedure for datatransmission in connection with pressure sensors, preferably inrestraining systems, is outlined on the basis of flow charts.

DETAILED DESCRIPTION

[0016]FIG. 1 shows a central control unit 10 which is connected todecentralized sensors 12, 14, 16, 18, 20, 22. In this case, the sensorseach include a sensor element (cf. 12 a, etc.) and an interfacecomponent (cf. 12 b, etc.), which is implemented in the preferredexemplary embodiment as an ASIC and in which the procedure for datatransmission described in the following is implemented as a program orsequential control. The data transmission from the sensors to thecentral control unit, which is unidirectional in the preferred exemplaryembodiment, is performed via two-wire lines 24 to 34, which areconnected to central control unit 10 and the decentralized sensors inthe framework of a point-to-point connection. In other exemplaryembodiments, a bus connection is used.

[0017] A preferred application of the system shown in FIG. 1 is inconnection with restraining systems for automobiles, it being possibleto vary the number of sensors from that shown in FIG. 1. In this case,the sensors are pressure sensors which detect a pressure exerted onthem. It is not possible for one single pressure sensor to analyzewhether the pressure information obtained is correct on the basis ofthis measured absolute pressure. In a system designed as in FIG. 1, thecentral control unit has multiple sets of pressure information availablefrom multiple pressure sensors, which do not communicate with oneanother, but do transmit all of their pressure values to the centralcontrol unit. Therefore, the control unit is capable, on the basis ofthe absolute pressure values of the individual sensors, of comparingthese with one another for plausibility and deriving error functions.Since the pressure gradient within the system, in particular within amotor vehicle, is negligibly small, the absolute pressure values of theindividual sensors must lie in a defined tolerance band relative to oneanother in the event of correct function. Therefore, for restrainingsystems, the absolute pressure values may be compared on this basis andchecked for plausibility in the central control unit.

[0018] A possible procedure for implementing this plausibility check isshown on the basis of the flow chart in FIG. 3. This flow chart outlinesthe program of a microcomputer contained in central control unit 10. Theprogram is run through at a specified cycle rate. First, in step 100,absolute measured values PABS1 to PABSn (n is the number of sensors)provided by the sensors are input. A pressure reference value Pref isthen produced in step 102. Depending on the embodiment, this value iseither one of the sensor pressure values or an average value of sensorpressure values, etc. In subsequent step 104, the individual pressuresensor values are compared to the reference value. Subsequently, it ischecked in step 106 whether there is an unacceptable deviation between asensor value and the reference value. If so, an error is established inthe relevant pressure sensor in step 108, while if there is nounacceptable deviation, i.e., if all sensor signals lie within apredetermined tolerance band, fault-free operation is assumed.

[0019] Other procedures, e.g., comparisons of the pressure values to oneanother, are also used in other embodiments for the plausibility check.

[0020] It has been shown to be suitable for many applications not totransmit absolute pressure values for performing the control tasks, butrather to transmit differential pressure values, for example thedifferential value between a reference pressure and the instantaneousmeasured pressure. In this way, environmental parameters are alreadytaken into consideration in the sensor, so that the measured pressurevalues transmitted do not have to be additionally analyzed in thecentral control unit. In these cases, it must also be ensured in thescope of the function check of the sensors described above that theabsolute pressure values are transmitted in addition to or instead ofthe differential pressure.

[0021] In a first exemplary embodiment, it has been shown to be suitableto perform the absolute pressure transmission in the initializationphase, and to change over to a differential pressure transmission afterthe initialization phase has ended. In this case, the sensors aretherefore checked in the initialization phase on the basis of theabsolute pressure values transmitted.

[0022] This embodiment is outlined on the basis of the flow chart inFIG. 2. This flow chart describes the sequence in the interfacecomponent of a pressure sensor. The measured pressure value is inputwhen the system is switched on (INIT phase) in first step 200. Thisvalue is then transmitted to the central control unit in step 202. It isthen checked in step 204 whether the initialization phase has ended. Ifnot, steps 200 and 202 are repeated. Otherwise, the measured pressurevalue is input in step 206 and differential pressure value Pdiff isproduced in step 208. This is performed, for example, by producing thedifference of the measured pressure value and a reference pressurevalue, for example an average of earlier measured pressure values. Thedifferential pressure value is then transmitted in step 210. Steps 206to 210 are repeated until the operating cycle is ended, by turning offthe supply voltage, for example. In this way, continuous transmission ofthe differential pressure value is ensured.

[0023] Depending on the exemplary embodiment, a point-to-point interfaceto each sensor or a bus system which connects all components is providedas the interface between the sensors and the central control unit. For apoint-to-point interface, it has been shown to be suitable in apreferred exemplary embodiment to provide a current-based two-wireinterface as outlined in the related art initially cited.

[0024] The first exemplary embodiment outlined above describes thetransmission of absolute pressure values in the initialization phase.Checking the sensor function during running operation is not possiblewith such an implementation. Therefore, permitting checking of theabsolute pressure value even during running operation, and thereforeallowing error detection in the sensor system in the way described aboveeven during running operation of the system, is provided as a supplementor alternative in the framework of a second exemplary embodiment.

[0025] If differential pressure values and/or normalized differentialpressure values are transmitted as described above, in order to beindependent of the ambient pressure and therefore independent of thecurrent elevation or pressure variations due to weather, measures are tobe taken which also allow absolute pressure values to be transmitted inaddition to these differential pressure values. In this case, normalizedpressure values are understood as pressure values which are normalizedin such a way that a signal not equal to 0 is transmitted only in theevent of dynamic pressure variations. This means that in the event ofstationary pressure values, the signal value is 0. In this case as well,errors in the sensor that are distinguished by an output signal which isconstant over time may not be recognized.

[0026] Therefore, mixing in absolute pressure values in addition totransmitting the normalized differential pressure values is providedduring transmission from the sensor to the central control unit duringnormal operation of the system. The transmitted absolute pressure valuesare then compared to one another by the central control unit asdescribed above and indices for correct and/or faulty function of theindividual sensors are derived therefrom.

[0027] The absolute pressure values are transmitted in this case only aslong as there is no significant signal change in the differentialpressure. As soon as such a change in differential pressure isrecognized, the sensor immediately stops the possibly ongoing absolutepressure transmission and switches over to differential pressuretransmission. This measure ensures that no system performance is lost bythe additional transmission of absolute pressure values.

[0028] Furthermore, data mixing or confusion is not possible, since thedifferent types of data (absolute pressure, differential pressure) areuniquely assignable via their value range.

[0029] In the preferred exemplary embodiment, the interface between thesensor and the central control unit is a current-based two-wireinterface, as is known from the related art initially cited. In thiscase, the differentiation between absolute pressure data anddifferential pressure data is performed through a correspondingidentification of the data using different identification codes. Inaddition, the absolute pressure value is coded in a data word whosevalue range lies outside the value range of the useful data(differential pressure).

[0030] In other embodiments, one of the measures described issufficient.

[0031] The absolute pressure values therefore include a combination ofidentification code and data word, both the identification code and thedata word being located outside the data range of the differentialpressure values. In this way, it is ensured that the absolute pressurevalues may not be confused with regular differential pressure values.

[0032] In the preferred exemplary embodiment, the value range for datatransmission in this case is divided essentially into three parts asdescribed in the related art initially cited, a middle range, whichincludes the useful data, the differential pressure data in the presentcase, and the regions outside the useful data range, which includestatus reports, identification data, etc. To transmit the absolutepressure values and to prevent collisions, the absolute pressure valuesare prefixed by their own identifier and the absolute pressure valuesare transmitted as a data word which has a value range lying outside theuseful data signal, i.e., in the status report range, for example.Because of the small word length in these ranges, the absolute pressuredata word is divided and is transmitted in multiple portions, providedwith an appropriate identifier.

[0033] Absolute pressure value is understood as the instantaneousmeasured pressure value, while an average value of preceding measuredpressure values, which are set in relation to the instantaneous measuredpressure value, is used to produce the differential pressure value.

[0034]FIG. 4 shows a flow chart in which the sequence in the sensor fortransmitting differential pressure values and absolute pressure valuesduring running operation is outlined. The sequence shown in FIG. 4 isperformed in predetermined time intervals. First, instantaneous measuredpressure value PMess is input in first step 300. Subsequently, in step302, reference pressure value PREF is produced on the basis of precedingvalues and differential pressure value PDIFF is produced in step 304 onthe basis of the instantaneous measured pressure value and the referencepressure value. This value may be supplied with an additionalnormalization in one exemplary embodiment, which normalizes thedifferential pressure value to an average value from precedingdifferential pressure values, for example. Subsequently, in step 306, itis determined on the basis of the instantaneous differential pressurevalue and one or more preceding differential pressure values whetherthere is a significant signal change. If so, a possibly ongoingtransmission of absolute pressure values is stopped, the instantaneousdifferential pressure value in the first value range, which is availablefor data transmission, is coded as a data word, the correspondingidentifier is selected, and the identifier and data word are transmittedin step 310.

[0035] If no significant signal change was recognized in step 304, thenin step 312 absolute measured value PMESS is coded as a data word in asecond value range which is available for data transmission. If the wordwidth is restricted in the particular application, the data word isseparated into multiple sections, the corresponding identifier isselected, and the data transmission is performed in multiple sectionswhich include identifier and data word in step 314. The proceduredescribed is then repeated in the next time interval.

[0036] The procedure described is used in connection with restrainingsystems in particular, but may also be used in other systems havingdistributed pressure sensors.

[0037] Furthermore, the application of the procedure described using theexample of pressure sensors is not restricted to pressure sensors, butrather may be used anywhere where decentralized sensors of the same typetransmit differential values as useful data to a shared central controlunit without the possibility of checking themselves.

1-10. (Canceled)
 11. A method for transmitting data from a sensor to acontrol unit, comprising: causing the sensor to measure a variable;causing the sensor to transmit the variable measured by the sensor tothe control unit as differential values; and transmitting additionalabsolute measured values of the variable in at least one operatingstate.
 12. The method as recited in claim 11, wherein: the at least oneoperating state includes an initialization phase of a system thatincludes the sensor and the control unit.
 13. The method as recited inclaim 11, wherein: the variable includes a pressure value.
 14. Themethod as recited in claim 11, further comprising: providing a valuerange for data transmission; dividing the value range into multiplevalue ranges; coding the differential values in a first value range; andcoding the absolute measured values in a second value range.
 15. Themethod as recited in claim 11, further comprising: providing identifiersfor data transmission, the identifiers being prefixed to a particulardata word and differing with respect to differential data and absolutedata.
 16. The method as recited in claim 11, wherein: at least one ofthe variable and the absolute measured values are transmitted via acurrent-based two-wire interface.
 17. The method as recited in claim 11,wherein: the absolute measured values and the differential values aretransmitted during a running operation.
 18. A sensor, comprising: aninterface for transmitting data to a central control unit; anarrangement for detecting measured values; and a component that relatesthe detected measured values to a reference value to producedifferential measured values, the component transmitting one of: thedifferential measured values, and in at least one predeterminedoperating state, absolute measured values.
 19. The sensor as recited inclaim 18, wherein: the sensor includes a pressure sensor in arestraining system for an automobile.
 20. A control unit for arestraining system that includes distributed, decentralized sensors ofthe same type, comprising: an interface; and an arrangement forreceiving differential values and absolute values from the sensors viathe interface, wherein: a function of the restraining system isperformed on the basis of the differential values, and a function checkof the sensors is performed on the basis of the absolute values.